One method of treatment of diseased or otherwise damaged vasculature has traditionally been through the implantation of vascular stents and/or grafts to maintain patency of the vasculature. It has also been known to implant such devices in saphenous vein bypass grafts, either at the time of bypassing the coronary arteries, or at a later date when the saphenous vein graft becomes partially or totally occluded.
Although wire stents are generally acceptable for use in larger vessels, because of the generally reduced cross-sectional area available for blood flow in smaller vessels, the use of a wire stent often encroaches to an unacceptable extent within the lumen of the vessel, causing blood cell damage and possibly clotting. Similarly, stents which are formed of two or more overlapping helices present an encroachment problem into the lumens of smaller vessels, such as the carotid artery, coronary artery, etc. An additional problem with grafts fashioned from wire, is that it is difficult to reduce (e.g., through folding, radial compression or other reduction technique) the downsized versions to an acceptable profile for insertion through and placement in the smaller sized vessels.
Stents which are formed of a series of interconnected rings, with the rings being substantially perpendicular to the longitudinal axis of the stent are also known. Because of variations in the cross-sectional mass of this type of stent along the longitudinal axis, this type of stent will tend to buckle in the weakest locations, e.g., generally in the locations where the rings are interconnected.
Many varieties of stents and stent-grafts have been described, but include one or more of the drawbacks discussed above. Pinchuk, U.S. Pat. No. 5,163,958, discloses a helically wrapped, undulating wire stent coated with a layer of pyrolytic carbon. The wire stent includes a plurality of generally circumferential sections 22, which are formed from the same continuous, substantially helically wrapped, undulating length.
Lau et al., U.S. Pat. No. 5,421,955, discloses an expandable stent made of a plurality of radially expandable cylindrical elements interconnected by one or more interconnective elements. The cylindrical elements may be individually formed from undulating elements. The entire stent may be made from a single length of tubing.
Schnepp-Pesch et al., U.S. Pat. No. 5,354,309, discloses a stent including a memory alloy part which radially widens at a transition temperature that is above ambient temperature but below body temperature. The stent may include a helically wound wire, as shown in FIGS. 4a-4b.
Leveen et al., U.S. Pat. No. 4,820,298, discloses a flexible stent constructed of a helix made from medical thermoplastic. Adjacent loops of the helix are interconnected by elastomeric strands. This allows the stent to be stretched into a somewhat extended, linear configuration, and to resume its helical shape upon release of the stretching forces.
Lau et al., U.S. Pat. No. 5,514,154, discloses an expandable stent made of a plurality of individual radially expandable cylindrical elements interconnected by one or more interconnective elements. The cylindrical elements may be individually formed from undulating elements. The entire stent may be made from a single length of tubing. The cylindrical elements include radially outwardly extending anchoring projections which may increase the profile of the expanded stent.
In summary, various stents, such as those discussed above, have been described with varying degrees of success. What has been needed and is addressed by the present invention, is a stent which has a high degree of flexibility for advancement through torturous pathways of relatively small diameter, can be readily expanded, and has sufficient mechanical strength to maintain patency of the lumen into which it is implanted, while minimizing the amount of lumenal encroachment to reduce the thrombosis risk.